The NYU Women's Health Study (NYUWHS) cohort has played a leading role in elucidating the associations of estrogens and androgens with breast cancer, based on blood samples that were obtained prospectively in 1985-91 from over 14,000 healthy women of ages 35-65. We now team up with another cohort from the Northern Sweden Health and Disease Study in Umea to address questions about the roles of estrogen metabolites in breast cancer. The proposed grant period would extend the NYUWHS follow-up to about 19 years on average and would permit the accrual of nearly 1,000 incident breast cancer cases, while the Umea study will have over 600 cases. The follow-up and cancer case ascertainment rates have been high in both studies. The study will investigate how much levels of estrogen metabolites - which can be both estrogenic and genotoxic - affect breast cancer risk, and the degree to which functional polymorphisms in estrogen metabolism genes are predictive of estrogen metabolite levels and of breast cancer risk. We hypothesize that: Circulating levels of 16alpha-hydroxyestrone are positively associated with breast cancer risk, and the 2-hydroxyestrone to 16alpha-hydroxyestrone ratio is negatively associated with breast cancer; Genetic polymorphisms associated with altered activity of enzymes catalyzing 16alpha-hydroxylation (CYP3A4 and CYP3A5) and 4-hydroxylation (CYP1B1) are associated with breast cancer risk. Functional polymorphisms in the sulfotransferase and glucuronidase genes that diminish estrogen conjugation activity are associated with increased breast cancer risk. Over the last five years, the NYUWHS cohort has been the basis for investigations, with a series of collaborators, of numerous risk factors for various cancers (breast, colorectal, endometrial, ovarian) e.g., IGF-I and its binding proteins; organochlorines; serum carotenoids, phytoestrogens, folate and homocysteine; polymorphisms in luteinizing hormone and DNA repair genes. The availability of serum specimens, DNA, and lifestyle and dietary data collected prospectively, combined with the extended followup and increasing numbers of cancers, will allow the study to continue to foster the investigation of various biological risk factors for a range of cancers.